1. Field of the Invention
The present invention relates to a separation mechanism for separating and feeding paper sheet, in particular, a separation mechanism for separating and feeding the paper sheet which is provided in a scanner apparatus, a laser beam printer, a copying machine, etc, and capable of separating plural sheets of paper sheet by sheet and feeding separated individual paper.
2. Description of the Related Arts (Prior Arts)
In the recent years, paper separating/feeding apparatus in the image forming apparatus installed in the copying machine, etc. is provided with a paper separation mechanism for separating the fed paper sheet. This apparatus adopts the method of separating and feeding paper by partially bringing the manuscript document into slidable contact with the respective convex portions of the conveying roller and the separating roller as a paper separating mechanism different from that of separating one sheet from the plural sheets of the manuscript document by bringing the plural manuscript documents, etc. into slidable contact with the overall surface of the conveying roller and the separating roller.
For instance, the published specification of Japanese Laid-open Patent Publication No. 4-89732/1992 describes such a paper separating mechanism as shown in FIGS. 7a and 7b.
In FIGS. 7a and 7b, the reference numeral 1 represents a conveying roller, 2a separating roller, and both of the rollers 1 and 2, respectively, have convex portions 1a and 2a and concave portions 1b and 2b provided between the convex portions 1a and 2a, and further the both pitches of the respective convex portions 1a and 2a are almost equal.
The conveying roller 1 and the separating roller 2 oppose each other in a state of forming uneven steps such that the convex portions 1a, 2a and the concave portions 1b, 2b do not come into contact with each other.
The conveying roller 1 rotates in a positive direction for the paper feeding direction of the manuscript document P, while the separating roller 2 rotates in a negative (inverse) direction for the same. Further, the frictional coefficient of the above-mentioned conveying roller 1 is larger than that of the separating roller 2.
Regarding those rollers; the conveying roller 1 and the separating roller 2, the engagement amount (overlap amount) between the concave portion 1b or 2b and the convex portion 1a or 2a is adjusted by use of the adjusting means not shown in FIGS. 7a and 7b, in accordance with the paper thickness of the manuscript document.
When the above-mentioned amount is adjusted, the adjusting means sets the overlap amount of the manuscript document such that the overlap amount thereof turns out to be large while that of the thick manuscript document turns out to be small. If the adjustment is not performed in such way, in the case of employing the thin manuscript document sheet, the document sheet deforms and as a result the same escapes from the convex portions 1a and 2a of the rollers 1 and 2.
In such a situation, the document sheet having weak waist (easily bent) deforms largely, while the other document sheet having strong waist (rigid) does not deform so much compared with the case of employing the thin manuscript document sheet having weak waist.
In such separating mechanism as mentioned heretofore, when two or more sheets of manuscript document are fed for example, the document sheets advance into the space between the rollers 1 and 2.
However, since the frictional force between the manuscript document and the separating roller 2 rotating in an opposite direction to that of feeding paper is larger than the frictional force between both of the manuscript document sheets, all other document sheets excluding the upper-most side document sheet are pushed back to the separating roller 2. As a result, only the upper-most side document sheet brought into slidable contact with the conveying roller is fed. In such a manner, the document sheet can be prevented from being fed in a state of two or more sheets.
However, in such a conventional sheet feeding apparatus, since the diameters (the diameters of the convex portions 1a and 2a) of the conveying roller 1 and the separating roller 2 are approximately equal to each other, there arose a problem to be solved in the past that double (superposing) conveying of the manuscript document sheets could not be prevented sufficiently.
Namely, in case that the diameters of the rollers 1 and 2 are almost equal to each other, the advancing angles .alpha. of two manuscript documents (the angles formed in the sheet feeding direction and the tangent line between the rollers 1 and 2) are made almost equal to each other, and thereby two sheets of the manuscript document advance toward the space between the rollers 1 and 2 at the same time. At this time, since the frictional coefficient of the separating roller 2 is larger than that of the conveying roller 1, two sheets of document are fed in a state of coming into contact with each other without separating the document sheets advancing at the same time toward the space between the rollers 1 and 2 on some occasions.
For this reason, the double conveying of the manuscript document cannot be prevented sufficiently.
Further, since the engaging amount of the rollers 1 and 2 (the distance between the rollers 1 and 2) has to be adjusted at every time by the adjusting means in accordance with the thickness of the manuscript document, the adjustment work for adjusting the engaging amount requires much time and thereby the adjustment work cannot be done efficiently. It is a problem to be solved in the prior-art technology.
Further, since the respective pitches between the convex portions 1a and 2a of the paper feeding (conveying) roller 1 and the paper separating roller 2 are approximately equal to each other, the manuscript document sheet is entirely applied with a uniform contacting force in a direction perpendicular to that of feeding the document sheet by the action of those convex portions 1a and 2a. In consequence, in case that a skewing of the document sheet occurs at the time of feeding, the document sheet is conveyed in a state of skewing by the conveying roller disposed at the downstream side of the paper separating mechanism.
Furthermore, since the convex portions have the uniform pitch, the document sheet is uniformly bent in the direction perpendicular to that of conveying the document sheet. For this reason, the separated document sheet recovers its original state from being in the bent state in the sheet conveying direction toward the center portion, and never-the-less the document sheet's both ends keeps the state of banding as it is on some occasions, and thereby an ear folding, etc. occurs on both ends of the document sheet in the width direction thereof. Those are the problems to be solved.